Reactions of hydroxyl radical (·OH) and oxide radical anion (O·−) with 2-aminopurine in aqueous medium (original) (raw)
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Reaction of Hydroxyl Radicals with Azacytosines: A Pulse Radiolysis and Theoretical Study
The Journal of Physical Chemistry A, 2006
Pulse radiolysis and density functional theory (DFT) calculations at B3LYP/6-31+G(d,p) level have been carried out to probe the reaction of the water-derived hydroxyl radicals (• OH) with 5-azacytosine (5Ac) and 5-azacytidine (5Acyd) at near neutral and basic pH. A low percentage of nitrogen-centered oxidizing radicals, and a high percentage of non-oxidizing carbon-centered radicals were identified based on the reaction of transient intermediates with 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate), ABTS 2-. Theoretical calculations suggests that the N3 atom in 5Ac is the most reactive center as it is the main contributor of HOMO, whereas C5 atom is the prime donor for the HOMO of cytosine (Cyt) where the major addition site is C5. The order of stability of the adduct species were found to be C6-OH•5Ac • > C4-OH•5Ac • > N3-OH•5Ac • > N5-OH•5Ac • both in the gaseous and solution phase (using the PCM model) respectively due to the additions of • OH at C6, C4, N3, and N5 atoms. These additions occur in direct manner, without the intervention of any precursor complex formation. The possibility of a 1,2-hydrogen shift from the C6 to N5 in the nitrogencentered C6-OH•5Ac • radical is considered in order to account for the experimental observation of the high yield of non-oxidizing radicals, and found that such a conversion requires activation energy of about 32 kcal/mol, and hence this possibility is ruled out. The hydrogen abstraction reactions were assumed to occur from precursor complexes (hydrogen bonded complexes represented as S1, S2, S3, and S4) resulted from the electrostatic interactions of the lone pairs on the N3, N5, and O8 atoms with the incoming • OH radical. It was found that the conversion of these precursor complexes to their respective transition states has ample barrier heights, and it persists even when the effect of solvent is considered. It was also found that the formation of precursor complexes itself is highly endergonic in solution phase. Hence, the abstraction reactions will not occur in the present case. Finally, the time dependent density functional theory (TDDFT) calculations predicted an absorption maximum of 292 nm for the N3-OH•5Ac • adduct, which is close to the experimentally observed spectral maxima at 290 nm. Hence, it is assumed that the addition to the most reactive center N3, which results the N3-OH•5Ac • radical, occurs via a kinetically driven process.
Journal of the American Chemical Society, 1995
Dialkylaminium cation radical kinetics were studied directly by laser flash photolysis methods. Irradiation of N-hydroxypyridine-2-thione carbamates with 355-nm light gave dialkylaminyl radicals that were protonated to produce dialkylaminium cation radicals. Fragmentation of the N-ethyl-2,2-diphenylethylaminium cation radical (2A) and cyclizations of the N-methyl-5,5-diphenyl-4-pentenaminium cation radical (6A) and the N-methy1-6,6-diphenyl-5-hexenaminium cation radical (9A) were studied. In organic solvents, these reactions are several orders of magnitude faster than analogous reactions of their neutral dialkylaminyl radical precursors and faster than reactions of isostructural carbon radical analogs. In acetonitrile and in THF with carboxylic acids as proton sources, reactions of 2A and 9A displayed saturation kinetics from which rate constants for reaction and apparent equilibrium constants for protonation could be determined by regression analysis of observed rate constants as a function of acid normality. In ethanol with perchloric acid, 9A was completely protonated. In aqueous solutions, the observed kinetics for reactions of 2A and 9A were dependent on the pH of the solutions which permitted determinations of both the rate constants for the reactions and the pKa values for the dialkylaminium cation radicals. Large kinetic solvent effects and a counterion effect when oxalic acid was the proton source in acetonitrile were observed. Arrhenius functions for 2A in acetonitrile and in THF and for 9A in acetonitrile, THF, and ethanol were determined. A rate constant for the 5-exo cyclization of 6A in acetonitrile at 20 "C was estimated by comparing the apparent second-order rate constants for reactions of the aminyl radical precursors to 6A and 9A with various carboxylic acids; with strong carboxylic acids, rate limiting protonation for the precursor to 6A apparently occurred. Second-order rate constants at ambient temperature for reactions of octanethiol and triphenylstannane with radical 2A were determined directly, and that for reaction of thiophenol with radical 9A was determined by indirect methods; the stannane, with an electron rich hydrogen, reacts much more rapidly with the aminium cation radical than with carbon radicals, whereas the thiols react less rapidly. Aminium cation radicals are useful in synthesis,'-3 are known or purported intermediates in biological oxidations of amines by enzymes>-6 and are produced in chemical, electrochemical, and photochemical electron transfer reactions of Sophisticated kinetic and PKa studies of anilinium radical cation^,'^-'^ trialkylaminium cation radical^,^^-'^ and radical
The Journal of Physical Chemistry B, 2008
The efficiency of the chemical pathway of DNA repair is studied by time-resolved chemically induced dynamic nuclear polarization (CIDNP) using the model system containing guanosyl base radicals, and tryptophan as the electron donor. Radicals were generated photochemically by pulsed laser irradiation of a solution containing the photosensitizer 2,2′-dipyridyl, guanosine-5′-monophosphate, and N-acetyl tryptophan. Depending on the pH of the aqueous solution, four protonation states of the guanosyl radical are formed via electron or hydrogen atom transfer to the triplet excited dye. The rate constants of electron transfer from the amino acid to the guanosyl radical were determined by quantitative analysis of the CIDNP kinetics, which is very sensitive to the efficiency of radical reactions in the bulk, and rate constants vary from (1.0 (0.3) × 10 9 M-1 s-1 for the cation and dication radicals of the nucleotide to (1.2 (0.3) × 10 7 M-1 s-1 for the radical in its anionic form. They were found to be higher than the corresponding values for electron transfer in the case of N-acetyl tyrosine as the reducing agent.
Organic & Biomolecular Chemistry, 2006
Reaction of the free-radical diphenylpicrylhydrazyl (DPPH • ) with Trolox R (TrOH) was investigated in buffered hydroalcoholic media by using a stopped-flow system. DPPH • was reduced to the hydrazine analogue DPPH-H with a measured stoichiometry of about 2. DPPH-H was characterized by an acid-base equilibrium (pK a = 8.6). Time-resolved absorption spectra recorded with an excess of either TrOH or DPPH • indicated that no significant amount of the TrO • radical was accumulated. The TrO • radical formed in a first step further reacted quickly with DPPH • . For 1 : 1 ethanol-buffer mixtures at pH 7.4, the bimolecular rate constants of the first and second steps were 1.1 × 10 4 M −1 s −1 and 2 × 10 6 M −1 s −1 , respectively. A significant increase of the measured rate constant was observed for ethanol-buffer solutions as compared to ethanol. The rate was also increased at higher pH. A deuterium isotopic effect of 2.9 was measured. These data are discussed with regards to mechanisms involving either electron or proton exchange as rate determining steps in the reaction of DPPH • with Trolox R . The importance of solvent acidity control in investigation of antioxidant properties is outlined. ; Fax: +33 1 69 87 43 60 † Electronic supplementary information (ESI) available: Absorption spectra taken in pH 6.4 and 8.4 buffers and in pure ethanol. See
Radiation Physics and Chemistry, 2002
The reactions of e À aq ; H-atoms and some one-electron reducing radicals with 2-aminopyridine (2-AmPy) and 3aminopyridine (3-AmPy) were studied using pulse radiolysis technique. The initial electron adduct of 2-AmPy, was found to react with the parent molecule to give a dimer radical at pH 4.8 and 9 which has entirely different spectrum from the initial adduct. The yield of the dimer radical increased with increasing concentration of the parent compound. The equilibrium constant for dimer formation was found to be 198 and 142 dm 3 mol À1 , respectively, at pH 4.8 and 9. In the case of 3-AmPy however, dimer formation was not observed. H-atoms add to both the compounds giving H-adducts as in the case of pyridine. The radicals derived from alcohols were able to reduce 2-AmPy to give pyridinyl type radical. However, in the case of pyridine and 3-AmPy these radicals were unable to give pyridinyl radical but react at acidic pHs by addition to the pyridine ring giving non-reducing species.
Chemistry-a European Journal, 2001
In aqueous solution, enolether radical cations (EE . ) were generated by photoionization (l 222 nm) or by electron transfer to radiation-chemically produced oxidizing radicals. Like other radical cations, the EE . exhibit electrophilic reactivity with respect to nucleophiles such as water or phosphate as well as electron transfer reactivity, for example, towards one-electron reductants such as phenols, amines, vitamins C and E, and guanine nucleosides. The reactivity of these electron donors with the radical cation of cis-1,2dimethoxyethene . (DME . ) can be described by the Marcus equation with the reorganization energy l 16.5 kcal mol À1 . By equilibrating DME . with the redox standard 1,2,4-trimethoxybenzene, the reduction potential of DME . is determined to be 1.08 AE 0.02 V/NHE. The oxidizing power of the radical cation of 2,3-dihydrofuran, which can be considered a model for the enolether formed on strand breakage of DNA, is estimated to be in the range 1.27 ± 1.44 V/NHE.
Nature of the transient species formed in the pulse radiolysis of some thiourea derivatives
Journal of The Chemical Society-perkin Transactions 2, 1994
Reactions of e − aq , OH radicals and H atoms were studied with n-allylthiourea (NATU) using pulse radiolysis. Hydrated electrons reacted with NATU (k = 2.8×10 9 dm 3 mol −1 s −1 ) giving a transient species which did not have any significant absorption above 300 nm. It was found to transfer electrons to methyl viologen. At pH 6.8, the reduction potential of NATU has been determined to be −0.527 V versus NHE. At pH 6.8, OH radicals were found to react with NATU, giving a transient species having absorption maxima at 400-410 nm and continuously increasing absorption below 290 nm. Absorption at 400-410 nm was found to increase with parent concentration, from which the equilibrium constant for dimer radical cation formation has been estimated to be 4.9×10 3 dm 3 mol −1 . H atoms were found to react with NATU with a rate constant of 5 × 10 9 dm 3 mol −1 s −1 , giving a transient species having an absorption maximum at 310 nm, which has been assigned to H-atom addition to the double bond in the allyl group. Acetoneketyl radicals reacted with NATU at acidic pH values and the species formed underwent reaction with parent NATU molecule. Reaction of Cl .− 2 radicals (k = 4.6 × 10 9 dm 3 mol −1 s −1 ) at pH 1 was found to give a transient species with λ max at 400 nm. At the same pH, reaction of OH radicals also gave transient species, having a similar spectrum, but the yield was lower. This showed that OH radicals react with NATU by two mechanisms, viz., one-electron oxidation, as well as addition to the allylic double bond. From the absorbance values at 410 nm, it has been estimated that around 38% of the OH radicals abstract H atoms and the remaining 62% of the OH radicals add to the allylic double bond.